ClutterContent has the ability to dictate the layout of any
given actor, through the CLUTTER_REQUEST_CONTENT_SIZE request
mode.
However, there is no way for ClutterContent implementations
to notify their attached actors that the content size changed.
Add a new optional ClutterContent.invalidate_size() vfunc and
clutter_content_invalidate_size().
https://gitlab.gnome.org/GNOME/mutter/merge_requests/405